2 CHAPTER 1. 



smaller, and ultimately terminate in very minute hair-like tubes, called 

 capillary vessels or capillaries. These vessels abound in every vascular 

 structure, and from them each structure absorbs those special nutrient 

 particles which are required for its growth or repair. In the capillaries 

 the arterial or outward-bound system of the circulation ends. 



In these same capillaries the venous or return circulation commences. 

 Into them also are returned the waste and used-up products of the 

 system. Reversing the previous order the capillary veins now enlarge 

 and coalesce and carry back to the heart dark, purple venous blood, 

 charged with the waste matters of the tissues, and among these with 

 carbonic acid gas. 



The impure blood thus brought back by the veins is carried to the 

 right side of the heart, and thence at each contraction or beat of that 

 organ is forced into the lungs. 



The lungs contain an infinite number of cells, which through the 

 bronchiae and windpipe communicate with the outer air. On the outside 

 of these cells the blood-vessels are spread. The blood thus exposed to 

 the air takes up from it a portion of its oxygen, and gives off the 

 carbonic acid gas and various volatile organic matters which have 

 accumulated in it, as described above. 



Thus freed from impurity, and containing more oxygen, the blood 

 again becomes bright scarlet and adapted for the nutrition of the body ; 

 and in this state it is returned to the left side of the heart for renewed 

 circulation through the frame. From birth to death this process is 

 always going on. 



3. Effect of breath on the air. 



The impurities of the blood are thus transferred to the air. Air, 

 therefore, which has been much breathed in contains too little oxygen 

 and too much carbonic acid gas, and is besides loaded with the organic 

 impurities given off by the lungs and also by the skin. Hence it is 

 obvious that the condition of the blood and the health of the animal 

 depend very much on the purity of the air supplied to the lungs. 



4. Composition of the air. 



Pure air consists of about four-fifths of nitrogen and one-fifth of 

 oxygen with small proportions of carbonic acid gas, aqueous vapour, 

 ammonia, ozone, and other constituents. For the purpose of purifying 

 blood, oxygen is the chief useful part of air. Nitrogen adds largely to 

 its volume and dilutes the oxygen, which would otherwise be much too 

 active. Any excess of carbonic acid gas is unwholesome, and an addition 

 of '5 per cent, is sufficient to render air irrespirable. 



5. Circulation of the air. 



Nature undisturbed takes her own means, into which it is not 

 necessary here to enter, of restoring the purity of the air, or, in other 



